Grant Aerona 3 vs Daikin Altherma for Retrofitting a 1930s Semi

A 1930s semi with cavity-less 225mm solid brick walls loses roughly twice the heat per square metre of a 1990s cavity-filled wall, and that single fact governs everything that follows. The Grant Aerona 3 in its 10kW R32 guise and the Daikin Altherma 3 H HT in the 14kW version are both sold into this exact house type, but they are engineered around different flow temperatures. Grant tunes the Aerona 3 for a design flow of 45 to 50 degrees. Daikin’s High Temperature Altherma will hold 65 degrees at minus 7 outside without an immersion top-up. That gap decides whether you keep the existing radiators or replace half of them.

The flow temperature decision comes first

Run the Aerona 3 at 45 degrees and its seasonal coefficient of performance in a UK climate sits around 3.4 to 3.8 on the MCS-standardised SCOP figures Grant publishes. Push the Daikin HT unit to deliver 65 degrees and the same metric drops toward 2.5 to 2.9, because compressing R32 to that temperature costs work. On a semi with a measured heat loss of 8kW at design conditions, that difference is real money. At 3.6 SCOP the Aerona draws about 2,220kWh of electricity to deliver the heating load over a season; at 2.7 the Daikin draws closer to 2,960kWh. At 28p per kWh that is a running cost gap near 200 pounds a year, every year the unit operates.

The catch that Grant’s brochure phrases as low flow temperature efficiency is that 45 degrees only works if the emitters can dump the heat at 45 degrees. A double panel radiator sized for a 70 degree gas boiler outputs less than half its rated wattage at a 45 degree flow. So the low flow temperature saving is not free. It is prepaid in radiator upgrades. Daikin’s pitch is the mirror image: keep the radiators, pay the difference at the meter. Neither vendor states the trade in those terms, so it is worth doing the sum for your own house before a salesperson does it for you.

Radiator balancing is where the install actually succeeds or fails

Most underperforming heat pump retrofits do not fail at the unit. They fail because the flow is unbalanced across the emitters and the return water never drops far enough for the compressor to modulate down. A heat pump wants a wide temperature spread across the system and a slow, high-volume flow. That is the opposite of how most 1930s systems were commissioned for a gas boiler running short, hot cycles.

Balancing means setting the lockshield valve on every radiator so each one drops the flow by a similar amount, usually a 5 to 7 degree delta-T across the whole circuit. On a semi with eight radiators this is an afternoon of work with two clamp-on pipe thermometers and a slow, iterative crawl from the radiator nearest the pump to the farthest. The near radiators get their lockshields nearly closed; the far ones stay open. Get it wrong and the lounge radiator roars while the back bedroom stays cold, and the installer’s reflex is to raise the flow temperature to compensate, which quietly destroys the SCOP the Aerona 3 was chosen for in the first place.

On a solid-brick semi the balancing job is harder because room heat losses are so uneven. A north-facing gable room behind an uninsulated solid wall can lose 40 percent more than an identical room on the party wall between the two semis, which barely loses heat at all. Sizing and balancing have to reflect that room by room, not by floor area. This is the single most skipped step, and it is the one that separates a heat pump that costs what the SCOP predicted from one that runs 15 percent worse for its whole life.

The practical test after commissioning is simple. Run the system for two hours in cold weather, then walk the house with a thermometer. Every radiator should be warm along its full width and top to bottom. Cold bottom corners mean sludge or air; a cold far end means the lockshield is throttling flow too hard. A magnetic system filter fitted on the return, such as an Adey MagnaClean Professional 3 or a Fernox TF1 Omega, catches the iron oxide that 90 years of cast iron and steel radiators shed, and it needs cleaning within the first few months because a fresh heat pump circuit stirs up decades of settled sludge that a gas boiler tolerated but a plate heat exchanger will not.

Solid brick walls: internal insulation before the unit is sized

The wall is the biggest single heat loss on this house type, and insulating it internally changes the heat pump sizing before you buy anything. A 90mm insulated plasterboard laminate using a Kingspan Kooltherm K118 board takes a solid 225mm brick wall from a U-value near 2.1 down to around 0.30 W/m2K. On a semi with maybe 60 square metres of exposed external wall, that is the difference between an 8kW and a 5.5kW design heat loss, which in turn is the difference between the 10kW and 6kW Aerona 3, and a smaller unit modulates better and cycles less at mild temperatures.

Internal wall insulation on solid brick carries a moisture penalty that the board manufacturers underplay. Sealing the warm face of a brick wall shifts the dew point into the masonry, and without a proper vapour control layer and attention to cold bridges at floor joists and window reveals, you get interstitial condensation and, over years, damp and rot in embedded timber. Kingspan’s own installation detail for K118 specifies a continuous vapour barrier and taped joints; the failure mode is almost always a rushed reveal or a joist end left uninsulated. This is why the sequencing matters. Insulate first, then have the heat loss recalculated, then size the unit.

Loft insulation is the cheapest kilowatt

Before either heat pump, top the loft to 300mm of mineral wool. The current UK building regulations target is 270mm; going to 300mm costs a few extra rolls and takes a lofted semi from perhaps 0.16 down to 0.11 W/m2K on the roof plane. It is the lowest cost per kilowatt saved of anything discussed here, and it directly shrinks the unit you need to buy.

Weather compensation is what makes the low-flow unit pay off

The Aerona 3 supports weather compensation through its controller, and this is the setting that turns its low flow temperature advantage from theoretical into actual. Weather compensation reads an outdoor sensor and slides the flow temperature up and down against a curve, so at plus 10 outside the unit might run 32 degrees and only climb to 48 at minus 3. Because compressor efficiency rises steeply as flow temperature falls, a system spending most of a UK winter at 35 to 40 degrees runs a materially better real-world SCOP than the single design-point figure suggests.

Daikin’s Madoka controller and its Onecta app offer the same weather-compensated logic, but on the High Temperature unit the benefit is smaller because the whole curve sits higher. A weather-compensated 65 degree system still spends its coldest days at 65. Setting the curve is not automatic. The installer picks a starting slope, and the homeowner adjusts it over the first winter by nudging it down a degree or two at a time until rooms just fail to reach temperature, then backing off. Most installers set a conservative high curve and never return, which is why so many heat pumps run hotter and dearer than the paperwork promised. The compensation curve is the one control setting worth learning to adjust yourself.

Running the numbers on a real 1930s semi

Take a specific case: a three-bed semi, 95 square metres, solid brick, single glazed originally but now double glazed, loft at 100mm. Pre-retrofit heat loss at minus 3 design comes out near 9kW. Add 300mm loft insulation and 90mm K118 internal wall insulation on the two exposed elevations, and the recalculated loss falls to roughly 5.5kW.

At that point the 6kW Aerona 3 at a 45 degree flow becomes viable, provided the four largest radiators are swapped for double-panel double-convector replacements and the system is balanced to a 5 degree delta-T. Estimated seasonal electricity: around 4,500kWh total heating and hot water. The alternative, a 9kW Daikin Altherma HT keeping every radiator at 60 to 65 degrees with no wall insulation, avoids the 4,000 pound internal wall job and the radiator swaps but runs at a lower SCOP and burns closer to 6,200kWh a season for the same comfort.

So the comparison is not really Grant against Daikin. It is a capital-heavy fabric-first route with a cheaper-to-run low temperature unit, against a lighter-touch install with a higher bill for the life of the equipment. Over a 15 year unit life at current electricity prices, the running cost gap alone reaches into four figures, which is roughly what the internal wall insulation costs to install. Whether that swap pays back depends entirely on how long you intend to own the house, and on whether the moisture risk in the solid walls is managed well enough that the insulation lasts the full period rather than needing to come back off.

The question the brochures never answer is what happens to the fabric-first economics if electricity and gas prices converge, because the whole low-flow advantage is priced in kilowatt-hours saved.